1. Field of Invention
The present invention relates to a multi-voltage headphone driver circuit, which provides more flexibility by way of providing multiple voltages.
2. Description of Related Art
FIG. 1 shows a typical structure of a conventional two-channel headphone driver circuit. In order to obtain a maximum dynamic range, the output of such conventional headphone is biased at a nominal DC voltage, which typically is half of a supplied voltage VDD (VDD/2). However, the DC bias causes significant amount of current to flow into the headphone speaker 16 and 18; this may, in addition to unnecessary power consumption, cause the headphone and the headphone driver to be damaged. Hence, the circuit requires capacitors Cdec to isolate the DC bias, and the capacitance of the capacitor Cdec is considerably large (in the range of several hundred μF).
The output waveform of this conventional headphone is shown in FIG. 2, in which the peak amplitude is VDD, the valley is 0, and the average is VDD/2.
In view of the problems caused by the DC bias, an improvement as shown in FIG. 3 is proposed, in which the low operation level of the two operational amplifiers is set to −VDD, so that the DC bias of the output voltage becomes 0. This eliminates the requirement of the large capacitors, but the circuit requires two operation voltages VDD and −VDD, and it is undesired to provide an additional I/O port for an external input of −VDD. Thus, this prior art proposes a charge pump 10, which converts the supplied voltage VDD to −VDD, so that the circuit only requires one voltage supply. The output waveform of this conventional headphone is shown in FIG. 4, in which the peak amplitude is VDD, the valley is −VDD, and the average is 0.
The second headphone driver circuit described above has the drawbacks that, the amplitude becomes double, so that the devices need to sustain higher voltage; and the power consumption also becomes double.
There are pros and cons in both foregoing prior art, but they have one common drawback: lack of flexibility, that is, they can not adaptively meet various different requirements. Thus, it is desired to provide a headphone driver circuit having higher flexibility than both prior art; the headphone driver circuit does not require devices with a higher voltage rating, or does not require large capacitors, or does not require both.